In the adhesives art, there is a trend in various countries towards increased use of compositions which are essentially solvent free and non toxic. In particular, there is a tendency towards use of hot melt adhesives in the manufacture of paper reinforcing tapes, books, sanitary napkins and diapers. In order to be fully satisfactory as a paper adhesive, a hot melt adhesive must be capable of supply in a handleable form for convenient application as a hot melt, and in use must provide bonds of satisfactory strength, flexibility and heat resistance. Desirably also, the composition of the adhesive will be such that when paper materials bonded with the adhesive are subjected to a repulping operation in order to reclaim the paper, the composition will not adversely affect the repulping of the paper. Repulping of paper materials normally involves removal of insoluble materials, including metal clips and adhesives, and then mastication of the paper material in water followed by subsequent filtering of the slurry and drying of the raw paper on heated drum rollers. At present it is a practice to remove and reject adhesively bonded papers prior to repulping so as to ensure that the heated drum rollers are not contaminated. It is desirable therefore to provide an adhesive composition which is sufficiently soluble or dispersible under repulping conditions to permit removal of at least the adhesive components of the composition in the water to facilitate repulping of paper bonded with the composition without adversely affecting repulping. Similarly, it is desirable that bonds in cellulosic napkins and diapers will break down in copious quantities of water for ready disposal.
There is disclosed in U.S. Pat. No. 2,657,189 a hot melt adhesive composition for bookbinding comprising vinyl acetate-crotonic acid copolymer, which composition is dispersible in dilute aqueous alkali. An essential component of this adhesive is chlorinated diphenyl, which is regarded as undesirably toxic. The presence of free carboxylic groups of the copolymer capable of salt formation with a dilute alkali (such as sodium, potassium and ammonium hydroxides) and is said to promote dispersibility and solubility in water. However, in order to achieve successful dispersibility or solubility, one relies on the use in the paper repulping operation of a suitable quantity of the necessary dilute alkalies. It is therefore apparent that careful control of the alkalinity of the aqueous repulping medium is a critical requirement for proper dispersion of the adhesives disclosed in this patent. This careful control is not always possible, and is especially inappropriate in those cases where domestic waste is disposed of through domestic sewage systems as may be the case with diapers and napkins. In such cases it is important that the adhesive bond break down comparatively quickly on contact with quantities of town water.